Apparatus for controlling electrode adjustment during aluminum oxide reduction

ABSTRACT

An apparatus for controlling the electrolytic reduction of aluminum oxide by adjusting the depth of immersion of anode blocks suspended on an anode bridge in an electrolysis melt containing Al2O3, said apparatus being of the type comprising an electric voltage supply for generating a reference voltage which is proportionate with the momentary bath current, an adjusting device for the anode bridge, which adjusting device is controlled by the difference in voltage over the bath and the reference voltage, and a system for deriving, according to a chosen program, a command signal for the adjusting device from the aforementioned differential voltage and the periodic, short excitation of the adjusting device by means of this possible command signal, said apparatus characterized by the improvement that the apparatus comprises an integrator having an input and an output, a set of relays each preceded by a trigger circuit connectable to said output, and means for feeding the differential voltage of the bath voltage and the reference voltage to the integrator and for disconnecting the integrator from the set of relays in the non-excited condition of the adjusting device, and for disconnecting the feeding of said differential voltage to said integrator and connecting said integrator to said set of relays in the excited condition of the adjusting device, and means for feeding back to the input of the integrator the integrated signal from said output when the differential voltage is fed to said integrator.

lJnitedStates Patent Arts et al.

[54] APPARATUS FOR CONTROLLING ELECTRODE ADJUSTMENT DURING ALUMINUM OXIDE REDUCTION [72] Inventors: Mathieu Gerardus Henricus Arts; Jan

Anna Antonius Liicker, both of Delfzijl;

Maarten Groenenboom, Escharen, all of I Netherlands Aluminium Delfzijl N.V., The Hague, Netherlands 22 Filed: Dec. 22, 1969 21 Appl.No.: 887,299

[73] 'Assignee:

[ July4, 1972 Primary Examiner-John H. Mack Assistant Examiner-D. R. Valentine Attorney-Hall & Houghton [57] ABSTRACT An apparatus for controlling the electrolytic reduction of aluminum oxide by adjusting the depth of immersion of anode blocks suspended on an anode bridge in an electrolysis melt containing M 0 said apparatus being of the type comprising an electric voltage supply for generating a reference voltage which is proportionate with the momentary bath current, an adjusting device for the anode bridge, which adjusting device is controlled by the difference in voltage over the bath and the reference voltage, and a system for deriving, according to a chosen program, a command signal for the adjusting device from the aforementioned differential voltage and the periodic, short excitation of the adjusting device by means of this possible command signal, said apparatus characterized by the improvement that the apparatus comprises an integrator having an input and an output, a set of relays each preceded by a trigger circuit connectable to said output, and means for feeding the differential voltage of the bath voltage and the reference voltage to the integrator and for disconnecting the integrator from the set of relays in the non-excited condition of the adjusting device, and for disconnecting the feeding of said differential voltage to said integrator and connecting said integrator to said set of relays in the excited condition of the adjusting device, and means for feeding back to the input of the integrator the integrated signal from said output when the differential voltage is fed to said integrator.

4 Claims, 3 Drawing Figures PATENTEDJUL "4 m2 3, 574,574

SHEET 1 OF 2 III INVENTORS BY M #704 ATTORNEY P'A'TENTEDJUL 4 I972 SHEEI 2 OF 2 UJE.

6\a m\ i Baa 1 INVENTORS BY W1 ATTORNEY APPARATUS FOR CONTROLLING ELECTRODE ADJUSTMENT DURING ALUMINUM OXIDE REDUCTION The invention relates to a control apparatus for controlling, for example, the electrolytic reduction of aluminum oxide by adjusting the depth of immersion of anode blocks suspended from an anode bridge in an electrolysis melt containing aluminum oxide, which apparatus comprises an electric voltage supply for generating a reference voltage proportional with the momentary bath current, an adjusting device for the anode bridge, which device is controlled by the difference in voltage over the bath and the reference voltage, and a system for deriving, according to a predetermined program, a command signal for the adjusting device from the aforementioned differential voltage and the periodic, short excitation of the adjusting device by means of this possible command signal.

- The invention further relates to an installation for the elec-.

trolytic reduction of aluminum oxide which comprises several electrolysis baths connected in series.

Although the invention is hereinafter elucidated with reference to the control of the process of the electrolytic reduction of aluminum oxide, it is not restricted thereto. Actually, the invention may be considered applicable to all those processes in which the process parameters vary as slowly as those in the process of the electrolytic reduction of aluminum oxide, and in which these process parameters and the adjusting signals for the process may be converted to, or from, electric signals respectively. In this instance particular consideration is given to various electrochemical processes, though the invention is definitely not limited to these.

In reducing aluminum oxide to metallic aluminum, it is common practice for an electrolytic process to be used, an electric current being passed through a solution of aluminum oxide in cryolite. The aluminum oxide A1 is then added to the bath in the form of alum earth.

By cryolite is meant, in this connection, 3NaF-AlF The electrolysis melt is present in a metal tank having a refractory lining, which, at least on the bottom of the tank, consists of carbon blocks forming the cathode. The anode is suspended into the bath, which anode is formed by a number of carbon blocks, all of which have been suspended from a so-called anode bridge.

The electrolysis of the aluminum oxide occurs as a result of the current passing through, which current also serves to maintain the temperature of the electrolysis bath at a constant level. The bath current required for the purpose may amount to about 100 to I kilogram-amperes with a total voltage drop over the bath U of 4 Volt odd.

In order to have the process proceed well, the temperature of the electrolysis bath is preferably maintained at a constant level. This may be achieved by maintaining the power input to the bath at a level which is as constant as possible.

In practice there are a great number of electrolysis baths electrically connected in series. Inasmuch as the internal resistance and to a lesser degree the polarization voltage too, differs from bath to bath, it is not possible to adjust the power input per electrolysis bath with the current passing through this bath. Consequently this adjustment has to be obtained to a great extent by adjusting the total resistance between the inleads and the out put leads of the bath, which may be effected by adjusting the distance between the anodes and the cathode. It may be remarked that the total voltage drop over the bath is built up from, among others, the following components (based on an electrolysis current of l00-l20.000 A) the ohmic voltage drop in the anode circuit together the ohmic voltage drop in the cathode circuit about 2.5 volt the ohmic voltage drop in the process the counter E.M.F. of the process (=polarization voltage) about 1.6 volt.

As the resistances in the bath, in the cathode and in the anode and the polarization voltage are process quantities which do not fluctuate rapidly and to a great extent, and inasmuch as the fluctuations in the current flowing through the bath are substantially slight, the process may essentially be controlled by varying the bath resistance, that is the height of the anode bridge. Variations in height of the anode bridge correspond with variations in depth of immersion of the anodes in the melt.

The object of this control is essentially to maintain the ohmic voltage drop in the process at a level that is as constant as possible, here, however, the so-called anodic phenomenon occurs as a complication. This anodic phenomenon, the origin of which is not quite understood, shows in a suddenly increasing resistance of the bath which periodically occurs. During this occurrence the bath voltage may very rapidly increase to 20 to 60 Volt.

This anodic phenomenon may be climinated by, for instance, breaking the crust on the bath and adding fresh alum earth to the bath.

The anodic phenomenon described above complicates the control of the process. If a control should react to each increase and decrease respectively of the bath resistance, the anode would be passed nearly into the metallic aluminum, or reversibly be lifted nearly out of the bath when an anodic phenomenon or a similar serious disturbance of the normal process should occur. In both cases impermissible complications will arise in the process, in the former case turbulent bath currents will occur resulting in a possible re-oxidization of the aluminum. In the second case, the power will be cut off I in the other electrolysis baths as well.

Therefore, the control system for a process like the electrolytical reduction of aluminum oxide should be able to meet the following demands, it should react on plain deviations in the desired process conditions, but should not react on rapidly occurring and/or substantial deviations in these process conditions. Another demand to be met for quiet proceedings of the process is that the control system should not react on deviations in the process conditions that are so slight that to adjust these deviations would only have a disturbing influence on the course of the process.

By not reacting to such slight deviations the life of the electro-magnetic components and of the electric and mechanic drive of the anode bridge is considerably lengthened as well.

It has already been proposed to control the electrolysis process of aluminum oxide into aluminum in the abovementioned way. It was suggested to connect the controller to an adjustable electrolysis bath for short periods of time during successive electrolysis periods and to supply controlling impulses as required as long as the determined deviation of the terminal voltage from the required voltage has not increased to a given value, but to supply a single controlling impulse only when this value has been exceeded, and finally not to supply any controlling impulse at all when a still greater deviation from the predetermined terminal voltage is achieved. The latter measure serves to prevent an anodic phenomenon from leading to a radical adjustment of the anode bridge.

It has been found that this prior art control unit may be improved upon. The fact of the matter is that this unit controls the process by the bath voltage which is measured during the short period-short with respect to the time constant of the bath-in the bath in which the control unit is connected to the electrolysis bath. It has been found that at a fairly low voltage level the bath voltage may change in magnitude rapidly. Accordingly, the control unit reacts repeatedly to deviations which last an extremely short time only, which makes that the control of the process proceeds in an unnecessarily unquiet way. In order to allow the process to make good progress, it is desirable, however, to have it proceed along an even course.

Moreover for economic reasons it has been found desirable in the chosen arrangement to have a single controller for a great number of baths connected in series. This may cause complications to occur. For in this case it is necessary to lay cables leading to the controller along the entire length of the installation, which cables have greatly different voltages. A stepping switch is required for this arrangement.

It is understood that an installation is more susceptible to failures if several baths are controlled by one and the same control unit. For, if a great many baths are controlled by one and the same control unit, a failure in this control unit will result in a considerable loss in production.

It is the object of the present invention to obviate these disadvantages of the control unit described above. With the unit according to the invention a more reliable and smoother run of affairs is ensured. The invention consists in that the unit for forming a command signal for the adjusting device according to a chosen program comprises both an integrator and a set of relays, each preceded by a trigger circuit, in which, in the condition of non-excitation of the adjusting device, the differential voltage of the voltage over the bath and the reference voltage is fed to the integrator and the latter circuit is disconnected from the relays, and in the condition of excitation of the adjusting device the integrator may be disconnected from the aforesaid differential voltage, but may be connected to the relays. The term integrator stands in this connection for both the linear and the non-linear time integrating circuits. This integrator being connected such that it is possible during the relatively long period in which the adjusting device is not excited to translate the deviation of the voltage over the bath with respect to the reference voltage into an average deviation in the preceding period, this average deviation serving to determine to which extent the adjusting device has to be actuated during the short period in which the adjusting device is excited. This forms as it were an imitation of the bath voltage to which is controlled instead of to the bath voltage itself. Because of this the control follows a much smoother course by which, on the one hand, the total number of adjusting motions of the anode bridge is limited to a considerable extent, and, on the other hand, a favorable effect on the course of the process is achieved. It has been found, in fact, that with the aid of this control unit a higher yield of aluminum may be obtained from the electrolysis baths.

It is further observed that only one single adjusting command is supplied per correction of the position of the anode bridge instead of one of a series, and this independent of the excitation period of the adjusting device. A further advantage consists in that the proposed control unit makes it also possible for deviations in the average error voltage to perform upward and downward adjusting motions of different magnitudes.

Furthermore it has been found that this control system and the corresponding external cables can be cheaper than the formerly used ones, this makes it no longer necessary for a collective control unit to be used. In this instance each bath may be controlled separately and this renders the use of a stepping switch superfluous, and furthermore results in the fact that the resistance of the direct current system with respect to earth is not unnecessarily decreased by the measuring lines.

It has already been observed that the integrator is arranged such that the deviation of the voltage over the bath with respect to the reference voltage is translated into an average deviation.

This may be achieved according to the invention by feeding the integrated signal back to the input of the integrator when the differential voltage is connected to the integrator.

If during the adjusting period" the command signal from the integrator would remain constant, the adjusting motion would continue indefinitely. To prevent this from happening, the duration of the adjusting signal is fixed depending on the strength of the output signal from the integrator. Precautions have to be taken for the output signal from the integrator to remain in existence as initial value during the following period in which the integrator is connected to the differential voltage.

A possible solution might be found in feeding back the bath voltage to the input of the integrator when the anode bridge is being adjusted, thus reducing the output signal of this integrating circuit to zero when the correction is made. A disadvantage of this solution is that during the adjustment of the anode bridge the bath voltage is still subjected to quite considerable, random fluctuations, because of which the output signal of the integrating circuit does not definitely drop back to zero.

By allowing the system a greater time constant this problem might be solved, but now it becomes apparent, however, that the system becomes unstable. These disadvantages are obviated according to the invention if during the connection of the integrating circuit to the relays, the output adjusting signal of these relays is fed back to the input of the integrating circuit transformed in such a way that by this feed back a simulation of the bath voltage correction in the integrating circuit is obtained. It has been found that in this way the output signal of the integrating circuit approaches zero in a stable way. The integrator thus forms at the moment of adjusting the anode bridge a kind of limitation of the actual bath, as it has shown itself during a short preceding period.

The unit according to the invention makes it possible for differentiations to be made between normal" deviations in the desired process conditions and suddenly occurring, great disturbances in the process conditions, as may occur when a so-called anodic phenomenon presents itself. The unit according to the invention is characterized in that there are four relays, two relays exciting the adjusting device for making an upward and downward motion respectively, if the corresponding associated trigger circuits are presented a voltage of corresponding sign by the integrator, which voltage exceeds a first threshold value, each of the two remaining relays blocking the motion mechanism if the associated trigger circuit is presented a positive and negative voltage respectively which exceeds a second greater threshold value. By choosing the first threshold values at i 0.05 Volt, and the second threshold values at t 0.5 Volt, for instance, it means that with a desired bath voltage of 4 Volt, for instance, the control unit will only interfere if the bath voltage over the reading period has been at an average of between 3.5 and 4.5 Volt. There will not be interfered either if the bath current amounts to less that 50 percent of the nominal value. It is understood that during such an averaging period voltage peaks of more than half a Volt may occur, but if these were of short duration, they will not effect the control of the process.

it has already been observed that a disadvantage of the prior art system is that it is necessary on economic grounds to utilize a single control unit for a whole series of electrolysis baths. The control unit according to the invention is eminently suitable to be connected to a single electrolysis bath or a small number of such baths, however. Because of this, long measuring lines which have been laid between the electrolysis baths and the control unit and are high-voltage lines have become superfluous as well. Taken by itself, it would also be possible for the control unit to be galvanically connected to the electrolysis bath. In practice, however, it is preferred for the control unit to be adapted to be earthed. It is therefore preferable according to the invention for a measuring transductor to be connected between the control unit and the bath voltage, said measuring transductor having a very high insulating resistance.

Finally, the invention further relates to an installation for the electrolytic reduction of aluminum oxide, which installation comprises several electrolysis baths connected in series, which have been connected to one control unit of the type described above.

The invention will now be elucidated further with reference to three figures.

FIG. 1 shows a diagrammatic representation of an electrolysis bath, with next to it a schematic indication of the way in which the voltage has been built up over this bath.

FIG. 2 explains with reference to a block diagram how the control unit operates.

FIG. 3 shows a block diagram of the control unit worked out in more detail.

In FIG. 1 the reference numeral 1 indicates the wall of a tank which contains the electrolysis bath. This metal wall has an internal lining of refractory material. The bottom 2 of this tank is lined on the inside with carbon bricks and forms the cathode of the electrolysis bath. Suspended on an anode bridge 3, which is maintained at a positive voltage with respect to the cathode, are a number of carbon blocks 4 which form the actual anode. At the bottom of the bath there is a layer of liquid aluminum 5 which is precipitated from the electrolyte 6 which consists of aluminum oxide dissolved in cryolite. On top of the bath is a crust 7, basically consisting of aluminum oxide (alumearth).

On the right-hand side of FIG. 1 is diagrammatically shown how the total bath voltage U, of about 4.1 ,Volt is built up from contributions of the anode resistance R,,, the bath resistance R the polarization voltage E and the cathode resistance 11,.

During the process the anode blocks will burn-off at the bottom, causing the distance between these blocks and the layer of aluminum to increase. Because of this, the bath voltage will increase and thus the power input of the bath. In order to prevent the temperature of the bath from increasing in an undesired way, the anode bridge has to be moved downwards. Because of this, the distance between the anode blocks being consumed and the layer of aluminum is essentially maintained at a constant level. In practice it has been found, however, that various factors, such as the aforementioned anodic phenomenon, may have a disturbing effect on this course of affairs. As a matter of fact, the distance between the anode blocks and the layer of liquid aluminum would be decreased too far, if a controller were to react on the anodic phenomenon.

Aluminum is periodically removed from the layer 5 by means of a suction pipe which is inserted at the top and reaches to the bottom of the bath. The quantity of aluminum oxide in the bath is kept at a constant level by periodically adding alumearth and simultaneously breaking the crust 7.

In FIG. 2 a diagrammatic representation is given of the control unit according to the invention. On the right-hand side of the figure U indicates the measured bath voltage, while on the left-hand side of the figure a reference voltage U,, is mentioned which is built up from two components in a way to be explained hereinafter. The first of these components is approximately equal to the polarization voltage in the bath, while the second of these components, proportionate with the bath current measured at any given moment, imitates the voltage drop in the anode, the cathode and the bath during a desired process. Line 8 indicates in what way the actual bath voltage is fed back to the reference bath voltage, and in that way a differential voltage U, is formed which is fed to a switch 9. In the position shown in the drawing the switch 9 feeds this differential voltage as input signal to an integrator 10, the output signal U A of which is connected to a switch 11. In the position shown in the drawing this output signal is fed back to the input of the integrator 10 by the switch 11. In this position shown in the drawing the circuit is in the period during which no command signal can be passed to theadjusting device, in this instance an electromotor not shown in the drawing, of the anode bridge. If the switches 9 and 11 are changed over to their other positions for a short period, the integrator is connected to a system of relays l2 and disconnected from the differential voltage U,'.

Depending on the nature of the signal U fed to the relays, these relays will transmit a command to have the anode bridge raised or lowered, or will not transmit a signal to the lifting device at all. By the reference numeral 13 the excitation of the adjusting device and the process in the electrolysis bath inclusive are diagrammatically shown. The outgoing signal thereof is a corrected bath voltage U With line 14 a feed back in this position of the switches 9 and I1 is shown, causing the adjusting signal to be fed to the integrator as input signal from the relays.

By a right choice of the various switching values a simulation of the process during the correction may be achieved therewith, with the result that at the moment that the bath voltage has attained the desired value, the excitation of the relays comes to a stop.

As will be explained hereinafter, the relays have been provided with so-called trigger circuits, so that these will only pass a command signal to the adjusting device if the absolute value of the voltage U, comes within predetermined limits.

In FIG. 3 the block diagram of FIG. 2 is worked out in more detail. Here, the amplifier A, fed back with the capacitor C, takes the place of the integrator from FIG. 2.

For safetys sake a galvanic isolation is required in the bath voltage measuring. This is the function of the measuring trans ductor T,. The circuit being dimensioned such that the D.C. voltage transformation is l 1.

The reference voltage U,, is composed of the adjustable voltages E and I R,,. The voltage E is derived from the stabilized supply voltage. The voltage I R,, is derived from an alternating current i,, which is proportionate to the bath current which may be provided centrally.

The operational amplifier A, has a very great amplifying factor, so that regardless of the output voltage, the input voltage may always be supposed to be equal to zero. There is therefore no input current either. If the amplifier is controlled at the negative input, the output signal has changed its sign. The voltages U and U,, are converted to currents by the resistors R1 and R2 and supplied to the input of the amplifier. If U,; and U,, are unequal, the differential current has to be supplied by the output of the amplifier via the feed back circuit (Rs,C1).

The fed back amplifier acts as a low-pass filter with a D.C. voltage amplification of 20 X and a time constant of 20 seconds.

The contacts of the relays R, A to D inclusive supply the in formation for controlling and signalling. Each relay is preceded by a trigger circuit which determines at which values of the presented voltage the relay operates or drops out. These values have been given in the diagram; the drop out voltage is placed between brackets.

The joint input of the triggers Tk.A and Tk.B at predetermined periods is connected to the outputs of the amplifier A, by the impulse contact S. If, for instance, the relay R,,.A operates, the function of S is taken over by the contact a,.

Although for the sake of clarity the term contacts is used in this description, in actual fact it is effected electronically.

When R .A operates, the contact a, opens as well. The amplifier is now fed back via C, only and is presented a constant current via P and R which is derived from the output voltage of the trigger Tk.A. The output voltage of the amplifier thus decreases linear with the time. When the value zero is reached, the relay drops out, and the original situation is resumed.

The triggers Tk.C and Tk.D react when the bath voltage deviation is more than 0.5 Volt of the desired value. They block the corresponding triggers Tk.A or Tk.B. This safety device blocks, among others, the controller when an anodic phenomenon occurs. This is signalled as well. When a failure should occur in the reference circuit or the measuring circuit, the controller is blocked too.

Since the bath voltage is occasionally manually corrected, the momentary value has got to be read. For this purpose the reference voltage is connected to a moving coil galvanometer via the operational amplifier A The amplifier prevents overloading of the reference circuit and also supplies an accurate zero offset of 2 Volt. The parallel connection of R and R-, has an equivalent value of 200 Ohm, so that the meter current varies with 5 mA/V.

The controlling command P is blocked when the reference voltage decreases to 2.8 Volt.

This happens when the bath current drops to less than half the nominal value. Occasionally, the bath current is switched ofi for some time. The polarization voltage Ep is then still present just as the part E, of the reference voltage. Inasmuch as the polarization voltage is independent of the anodecathode cathode distance, the controller might send out meaningless, and as such undesirable, adjusting commands.

We claim:

1. Apparatus for controlling the electrolytic reduction of aluminum oxide by adjusting the depth of immersion of anode blocks suspended on an anode bridge in an electrolysis melt containing A1 said apparatus being of the type comprising an electric voltage supply for generating a reference voltage which is proportionate with the momentary bath current, an adjusting device for the anode bridge,which adjusting device is controlled by the difference in voltage over the bath and the reference voltage, and a system for deriving, according to a chosen program, a command signal for the adjusting device from the aforementioned differential voltage and the periodic, short excitation of the adjusting device by means of this possible command signal, said apparatus characterized by the improvement that the apparatus comprises an integrator having an input and an output, a set of relays each preceded by a trigger circuit connectable to said output, and means for feeding the differential voltage of the bath voltage and the reference voltage to the integrator and for disconnecting the integrator from the set of relays in the non-excited condition of the adjusting device, and for disconnecting the feeding of said differential voltage to said integrator and connecting said integrator to said set of relays in the excited condition of the adjusting device, and means for feeding back to the input of the integrator the integrated signal from said output when the differential voltage is fed to said integrator.

2. Apparatus as claimed in claim 1, further characterized in that the apparatus comprises means by which, in the period during which the integrator is connected with the set of relays, the output adjusting signal of the set of relays is fed back to the input of the integrating circuit in such a form that by this feed back a simulation of the correction of the bath voltage in the integrator is achieved.

3. Apparatus as claimed in claim 1, further characterized in that said set of relays comprises four relays, two relays exciting the adjusting device for an upward and downward movement respectively, provided the associated corresponding trigger circuit is presented a voltage of corresponding sign by the integrator, which voltage exceeds a first threshold value, and each of the two remaining relays blocking the movement mechanism if the associated trigger circuit is presented a positive and negative voltage respectively which exceeds a second, higher threshold value.

4. Apparatus as claimed in claim 1, further characterized in that the apparatus further comprises a highly insulating transductor for coupling the bath voltage thereto. 

2. Apparatus as claimed in claim 1, further characterized in that the apparatus comprises means by which, in the period during which the integrator is connected with the set of relays, the output adjusting signal of the set of relays is fed back to the input of the integrating circuit in such a form that by this feed back a simulation of the correction of the bath voltage in the integrator is achieved.
 3. Apparatus as claimed in claim 1, further characterized in that said set of relays comprises four relays, two relays exciting the adjusting device for an upward and downward movement respectively, provided the associated corresponding trigger circuit is presented a voltage of corresponding sign by the integrator, which voltage exceeds a first threshold value, and each of the two remaining relays blocking the movement mechanism if the associated trigger circuit is presented a positive and negative voltage respectively which exceeds a second, higher threshold value.
 4. Apparatus as claimed in claim 1, further characterized in that the apparatus further comprises a highly insulating transductor for coupling the bath voltage thereto. 